Bondable coating on aluminum and method of applying it



United States Patent 3,414,489 BONDABLE COATING ON ALUMINUM AND METHOD OF APPLYING IT Narvel L. Rogers, Hurst, Tex., assignor to Bell Aerospace Corporation, Wheatfield, N.Y., a corporation of Delaware No Drawing. Filed Jan. 13, 1966, Ser. No. 520,344

8 Claims. (Cl. 204-38) This invention relates to a method for obtaining a coating on aluminum or aluminum alloys that is both corrosion-resistant and suitable for adhesive bonding. More particularly, the invention relates to a method for sealing an anodized aluminum surface and for providing the surface with a physical and chemical make-up receptive to organic adhesives.

One of the most common ways for preparing aluminum or aluminum alloys for adhesive bonding involves an acid etching process. This process usually consists of cleaning and rinsing the metal and thereafter immersing it in a solution of acid and water to which certain inhibitors, such as sodium dichromate, have been added. The metal is then thoroughly rinsed in clean water. It is believed that in such a process the hexavalent chromium is obsorbed on the metal surface so as to provide reactive sites for molecular bonding with adhesive media.

Although such known processes form good adhesive receptive surfaces, they produce a product that is highly susceptible to corrosion, particularly along the adhesivemetal interface. This naturally results in eventual destruction of the bond.

Another common aluminum coating process involves anodizing with chromic acid. In this process the cleaned and rinsed metal is immersed in a deoxidizer solution to remove oxides and scale. After deoxidizing and rinsing, the metal is anodized in an electrolyte solution containing chromic acid. Following the anodize treatment, the surfaces are rinsed in room temperature water and then dipped mementarily in clean water at a temperature not exceeding 150 F. This causes part of the surface to convert to corrosion-resistant aluminum oxide. Nonetheless, the aluminum OXide coating is itself fairly porous, and corrosive media may readily penetrate to the base metal. Although the coating can be sealed against corrosion by immersion in water of boiling temperature, this has been found to destroy the effectiveness of the surface for adhesive bonding.

It is therefore an object of the present invention to overcome the disadvantages associated with prior anodized aluminum products by employing a method that makes an aluminum or aluminum alloy surface both corrosionresistant and receptive to organic adhesives.

It is an object of the instant invention to provide a method that will effectively seal the corrosion-resistant anodize on aluminum or its alloys and also provide a surface that is physically and chemically receptive to bonding with adhesives.

It is a further object of the present invention to provide aluminum metal or its alloys with corrosive-resistant surfaces that have bonding characteristics that are equal to or better than those obtained by acid-etching processes.

Yet another object of the instant invention is to provide, in solution, a novel composition adapted to both seal an anodized aluminum surface and to render it receptive to organic adhesives.

The present invention involves a process for improving the corrosion-resistant and bonding qualities of anodized aluminum or its alloys by immersing it for several minutes in a seal solution maintained at a temperature of at least 180 F. The seal solution is highly important to the success of the process of the invention, and consists of a sol of highly pure water to which has been added about 50 to 200 parts per million by Weight (p.p.m.) of chromic acid (chromium trioxide).

It is known that a chromic acid solution wherein the chromic acid is 10 p.p.m. will ruin the seal. Similarly, it is known that a very highly concentrated chromic acid solution will also destroy the seal. Thus the essence of the present invention is in the teaching that an intermediate range of chromic acid concentration (and specifically a chromic acid concentration of between about 50 and 200 p.p.m.), if applied under proper conditions, is effective to treat an anodized aluminum surface without substantially weakening the seal to provide a surface that has good bonding qualities. By employing a chromic acid solution which has a limited range of concentration of chromic acid and is otherwise very pure, it becomes possible to treat the surface of an aluminum article so as to provide both corrosion-resistance and high adaptability to be bonded with organic adhesives.

The purity of the water used in preparing the seal solution is essential to the obtainance of good corrosionresistance. It should have a specific resistance at 30 C. of not less than 30,000 ohm/cm. and a total disassociated solids content of no more than 15 parts per million (p.p.m.). For best results, the water should conform to the following additional standards:

P.p.m. max. Total alkalinity (calc. as CaCO l0 Phenolphthalein alkalinity (calc. as CaCO 1 The seal soluton is made by adding chromic acid to water having the above purity characteristic. During use, this seal solution is maintained at a pH between about 2.5 and 3.7 at a temperature between about F. and F. The pH and the temperature of the seal solution in use are both important. When the pH falls below about 2.5, there is a marked tendency for the seal solution to remove the anodize. On the other hand, when the pH rises above about 3.7, there is a marked tendency for the bonding strength to weaken. Maintenance of pH outside these limits is possible, but experience shows that in such cases consistently good product quality cannot be obtained.

With regard to the temperature of the seal solution, when it is below about 180 F. the results are erratic insofar as corrosion-resistance is concerned. Moreover, when the seal solution temperature is above about 195 F. the results are erratic insofar as destroying the bonding effectiveness of the surface is concerned. Although it may sometimes be possible to deviate from this temperature range of between 180 F. and 195 F. and obtain acceptable results, experience has shown that the results are not reliable, and that the maintenance of the temperature to this range is therefore of major importance in obtaining a consistently good product.

The concentration of chromic acid has been found to be of considerable importance. A change in chromic acid concentration is somewhat related to the same effects as observed above with respect to the pH, since chromic acid is used for pH control. In any event, failure to keep the chromic acid concentration above about 50 p.p.m. results in a non-bondable surface, whereas a failure to keep the concentration below about 200 p.p.m. risks removal of the anodize.

However, if the seal solution is kept within the aforesaid limits with respect to pH, temperature and chromic acid concentration, there is obtained the new result of a surface on aluminum that is simultaneously protected against corrosion and readily bondable to common organic adhesives.

Although the theory behind the successful effects achieved by the seal solution is not known with certainty, the following is 'believed to take place: the hot l80l95 F. seal solution converts part of the aluminum oxide to aluminum oxide monohydrate. The conversion to the monohydrate form increases the size of the aluminum oxide particles and seals the pores in the oxide coating. This seal therefore forms a barrier that helps prevent the penetration of corrosive media to the base metal. The chromic acid content of the seal provides a minute dissolution of the exterior surface of the oxide coating and hexavalent chromium is absorbed on the surface. The combination of minute dissolution and absorbed chromium produce a strong molecular and mechanical bond with organic adhesives. The latter effect is achieved without jeopardizing the barrier against corrosive media.

The treatment according to the invention can be successfully carried out on aluminum or its alloys. Insofar as alloys are concerned it is best to use the process of the invention, including the novel seal and bonding solution, on those alloys whose copper content does not exceed about 5% or whose total alloying content does not exceed about 7.5%. By way of example, the following procedure utilizing the steps of the invention may be used to yield a corrosion-resistant adhesive-bondable surface on aluminum.

The metal is initially degreased and cleaned in a mild alkaline solution to remove soils. Either room temperature solvents or hot trichloroethylene, supplemented by an aluminum cleaner, are satisfactory for this conventional operation. The metal is thoroughly rinsed in warm clean water. Next, the metal is deoxidized to remove scale and oxides in the conventional manner by a solution of sulfuric acid with sodium dichromate inhibitor. This is followed by a clean water rinse.

Thereafter, the metal is connected as the anode in an electrolytic cell having a chromic acid electrolyte and a suitable cathode. The electrolyte suitably contains:

Percent Total chromic acid 610 Free chromic acid (min) 5.0

The aluminum anode is subjected to a potential of 40:2 volts D.C. for about 30-35 minutes at about 92-98 F. and at a current density of at least 1 amp/ft. so as to create .a tenacious surface coating of aluminum oxide.

The anodized metal is rinsed in clean room temperature water for one or two minutes and is thereafter immersed for about 7-9 minutes in a seal and bonding solution maintained at 180-195 F. and having the following make-up and characteristics:

pH 2.5-3.7 Chromic acid, p.p.m 50-200 Water sol:

Specific resistance at 30 C ohm/cm.

min 30,000 Total solids p.p.m. max" 15 Total alkalinity (calc. as CaCO p.p.m.

max 10 Phenolphthalein alkalinity (calc. as CaCO p.-p.m. max 1 The metal is removed from the seal and bonding solution, and air dried.

The time during which the anodized metal is immersed in the seal and bonding solution is of some importance. Experience has shown that seven minutes is about the minimum time of immersion for obtaining adequate surface treatment. Nine minutes has been found to provide adequate treatment for all the aluminum parts tested. Where relatively thick aluminum parts are involved, a seven minute time period might be too short a time period since there would not be enough time for the part to adequately warm up to the temperature of the solution. Thus, for the thicker parts, nine minutes might be the minimum time required and it would appear that nine minutes is an adequate time period for almost any aluminum part.

The bonding quality of products made in accordance with the above process are equal to or superior to those obtained by the standard acid etching procedure. Comparative tests for shear and peel were run on several representative bonded aluminum samples treated by the sealed anodize procedure of the invention and by the conventional hot acid etching technique. Both sets of test sheets were bonded under identical conditions with three different brands of nylon-modified epoxy resin. The results, all averages of several tests, are reported below:

1 P.l.i.pouuds per linear inch. 2 No evaluations made for shear.

As can be seen from the above table, the resistance of the bond to shear or peel is fairly comparable as between the two techniques. Both show good adhesive quality which varied slightly depending on the particular bonding agent used. As pointed out above, however, the acid etch method provides very little protection against corrosion, and it is the purpose of the instant invention to provide surfaces that are both corrosion-resistant and effective for bonding.

The effectiveness of the corrosion seal can be quickly tested in the following manner. A drop of a solution of 1 gram of anthroquinone violet in 50 ml. distilled water is placed on the surface of the metal and permitted to stand for 5 minutes. It is then washed with mild soap and water. If the surface is properly sealed, the dye will have been completely removed.

Although prior procedures could, under certain conditions, give equivalent sealing against corrosion, they fail, in contrast to the invention, to achieve surfaces that are acceptable for adhesive bonding.

Although the invention has been described as utilizing a chromic acid anodize, other anodizing electrolytes such as oxalic and sulfuric .acid are suitable. Indeed, there is no particularity as to the method by which the anodize coating is achieved so long as this is followed by the appropriate treatment of a seal and bonding solution prepared as disclosed herein.

Bonding .agents composed of standard organic adhesives are all'generally suitable although, relatively speaking, there are, of course, individual differences between them. Particularly useful bonding agents are the nylonmodified epoxy resins, standard epoxy, nitrile epoxy, epoxy novolac, phenolic and nitrile-phenolic resins, polysulfides, silicone adhesives, and the polyurethanes.

What I claim is:

1. A method for forming a corrosion-resistant adhesivebondable surface on aluminum and aluminum alloys, comprising cleaning and rinsing the metal surface, removing scale and oxides on the metal by subjecting it to deoxidizing conditions, anodizing the surface to form a coating of aluminum oxide, rinsing the anodized aluminum in room temperature water, thereafter immersing it for several minutes in a sealing and bonding solution maintained at about 195 F. and a pH of about 2.5 to 3.7; said sealing and bonding solution comprising high purity water to which has been added about 50 to 200 p.p.m. of chromic acid (chromium trioxide), and thereafter drying the treated aluminum.

2. The method of sealing anodized aluminum or its alloys against corrosion and making its surface receptive to organic adhesives, comprising the steps of: immersing the anodized aluminum in a solution comprising about 50 to 200 parts per million by weight of chromic acid dissolved in high purity water, said high purity water having a specific resistance of not less than 30,000 ohms/cm. at 30 C. and having a total solids content of not more than 15 parts per million by weight, while said solution is maintained at a temperature of about 180 F. to about 195 F. and at a pH of about 2.5 to about 3.7, and thereafter removing and drying said aluminum.

3. The process according to claim 2 wherein the high purity water has a total alkalinity no greater than about p.p.m. equivalents of calcium carbonate and a phenolphthalein alkalinity no greater than about 1 p.p.m. equivalents of calcium carbonate.

4. The process according to claim 2 wherein the anodized aluminum is immersed in the said solution for a period of at least seven minutes.

5. The new bondable, corrosion-resistant aluminum article prepared according to the method of claim 2.

6. A composition for sealing a porous anodized aluminum surface and rendering it receptive to organic adhesives, consisting essentially of a solution containing 50 to 200 p.p.m. chromic acid in high purity water, said high purity water having a specific resistance of at least 30,000 ohm/cm. at 30 C. and a total solids content of not more than p.p.m., and said solution having a pH of about 2.5 to 3.7.

7. The method of protecting the surfaces of aluminum and its alloys against corrosion and making such surfaces receptive to organic adhesives, comprising the steps of:

depositing an adsorptive oxide coating on the aluminum surface, thereafter impregnating it with a solution comprising to 200 p.p.m. chromic acid in high purity water, said high purity water having a specific resistance of not less than about 30,000 ohm/cm. at 30 C. and a total solids content of no more than 15 p.p.m., while the said solution is maintained at a temperature of about F. and at a pH of about 2.5 to 3.7, and thereafter drying said surface.

8. In the method for anodizing aluminum or its alloys comprising applying a voltage to an initially clean aluminum anode in an electrolytic cell having an electrolyte with free ions derived from the class consisting of oxalic acid, sulphuric acid and chromic acid, the improvement of providing the product of said anodizing process with a corrosive-resistant and adhesive bondable coating comprising rinsing the anodized aluminum in clean water at room temperature, then immersing it in a solution containing about 50 to 200 p.p.m. chromic acid in water having a specific resistance of not less than 30,000 ohms/cm. at 30 C. and a total solid-s content of no more than 15 p.p.m., said solution being maintained at a pH of about 2.5 to 3.7 and a temperature about 180-195 F., and thereafter drying the coated aluminum surface.

References Cited UNITED STATES PATENTS 2/ 1934 Edwards 148-62 5/1962 Michel 20438.42 

1. A METHOD FOR FORMING A CORROSION-RESISTANT ADHESIVEBONDABLE SURFACE ON ALUMINUM AND ALUMINUM ALLOYS, COMPRISING CLEANING AND RINSING THE METAL SURFACE, REMOVING SCALE AND OXIDES ON THE METAL BY SUBJECTING IT TO DEOXIDIZING CONDITIONS, ANODIZING THE SURFACE TO FORM A COATING OF ALUMINUM OXIDE, RINSING THE ANODIZED ALUMINUM IN ROOM TEMPERATURE WATER, THEREAFTER IMMERSING IT FOR SEVERAL MINUTES IN A SEALING AND BONDING SOLUTION MAINTAINED AT ABOUT 180-195*F. AND A PH OF ABOUT 2.5 TO 3.7; SAID SEALING AND BONDING SOLUTION COMPRISING HIGH PURITY WATER TO WHICH HAS BEEN ADDED ABOUT 50 TO 200 P.P.M. OF CHROMIC ACID (CHROMIUM TRIOXIDE), AND THEREAFTER DRYING THE TREATED ALUMINUM. 